The present invention relates to circuits for producing focus and tracking error signals to a servo system in an optical disk drive.
A typical optical disk system is shown in FIG. 1. An optical disk 12 which spins on a spindle 14 stores data which can be read and perhaps written with a laser beam. The data may be in the form of physical holes burned into a sensitive layer or layers on the disk substrate, or may be changes in the magnetic properties of the disk when exposed to the laser which cause a polarization of the reflected laser beam, or some other method. A laser diode 16 produces an elliptical, expanding laser beam which is converted into a parallel beam by a collimator 18. A beam-shaping prism 20 makes the elliptical beam circular, and the circular beam then passes through a beam-splitter 22, is reflected off a mirror 24 through a retardation plate 26 to an objective lens 28. Objective lens 28 focuses the beam on disk 12. A voice coil 30 serves to move objective lens 28 toward or away from disk 12 to keep it in focus. A second voice coil 31 can also move objective lens 28 sideways for tracking.
The reflected laser beam on the return path passes through objective lens 28, retardation plate 26, off mirror 24 and beam-splitter 22 to a second mirror 32 and a convergent lens 34 for focusing the reflected beam on a four-quadrant photodetector 36. A cylindrical lens 38 serves to produce an astigmatic image on photodetectors 36 when the beam is out of focus. The entire system from laser diode 16 through all of the lenses back to detector 36 is packaged as an optical head which is itself moved (or only the objective lens is moved) across disk 12 to track the data. This tracking is typically done by monitoring grooves which are stamped or molded into the surface of disk 12, with the data itself being either between the grooves or in the grooves themselves.
Photodetector 36 serves to detect data by monitoring variations in the intensity of the reflected laser beam. In addition, the use of four quadrants in photodetector 36 enables detection of focus errors and tracking errors. FIG. 2A shows the laser beam impinging equally on quadrants A, B, C and D of photodetector 36. Each of these quadrants is a separate photodetector producing a separate electrical signal. The sum of the outputs of the quadrants produces the read signal. When the beam is out of focus it will assume an elliptical shape as shown in the example of FIGS. 2B and 2C because the focal point is beyond or before the disk surface. In FIG. 2B, the distance between the objective lens and the disk is too small and in FIG. 2C the distance between the objective lens and the disk is too large (the opposite could be the case in a different system). Typically, a focus error signal is produced by calculating the difference between the light detected by quadrants A and C and the light detected by quadrants B and D. As can be seen, this value will be 0 for an in-focus condition, a negative value when the distance between the objective lens and the disk is too small and a positive value when the distance between the objective lens and the disk is too large.
A number of methods may be used for focus control. U.S. Pat. No. 4,556,965 to Tsunoda, et al., discusses a prior art method of using an air jet nozzle to keep an optical head a constant distance off of a disk.
U.S. Pat. No. 4,547,872 to Henmi, et al., produces a primary and two secondary laser beams with separate photodetectors. The amplified difference between the first and second secondary laser beams is used for focus control.
U.S. Pat. No. 4,532,522 to Tsunoda produces two additional light beams either both in front or both behind the primary light beam for tracking.
In U.S. Pat. No. 4,559,622 to Hazel, et al., the focus beam is split and sent to two photodetectors which are different distances from the optical disk. The beam will be parallel when the beam is in focus and the same amount of light will be on both photodetectors. A converging or diverging beam, indicating an out-of-focus condition, will result in different amounts of lights on the different photodetectors.
Finally, U.S. Pat. No. 4,556,965 to Tsunoda discusses the use of a four-quadrant photodetector such as that shown in FIG. 1. In this method, a single beam can be used for reading and for focus error detection.
FIGS. 3A-3C show the type of signal received in an interference track detection system. In such a system, if the light beam varies from the center of the track, the illumination on the left and right side of the photodetector will be different. One cause of the variation is the diffraction pattern generated by the grooves on the disk. Another cause of the variation is the light impinging upon a groove reflects back in all different directions and not just straight back to the photodetector. Yet another cause of variation is the inherent reflectivity difference of the area between the grooves and the area in the grooves. Accordingly, FIG. 3A shows an on-track condition, FIG. 3B shows the beam being off-track to the left such that quadrants A and D receive less light than quadrants B and C, and FIG. 3C shows the beam being off-track to the right. Accordingly, the tracking error signal is given by the difference between the light on the A and D quadrants and the light on the B and C quadrants.
Since the error signals produced are difference signals, they must be calibrated to the intensity of the laser beam and the reflectivity of the media which can cause equal variations in intensity across all four photodetectors. In addition, there can be unequal variations in intensity. The laser beam wavefront can have distortions or aberrations (which may vary with intensity), resulting in an uneven distribution across the four photodetectors, and data may cause unequal changes. If the modulation of light due to the detection of data is not even on all four quadrants, false errors may again be detected. These variations may differ with intensity as well. Also, the laser beam is typically not exactly centered on the quad photocell due to optical and mechanical tolerances. Unless these various offsets are compensated for, the size of the error signal generated will be larger or smaller than appropriate. This will affect the servo system offset and response (over or under correction) and may result in instability.
Changes in the intensity of the beam also occur during a write signal for an optical head which is capable of writing as well as reading. In one method of writing, a disk substrate which is only 4% reflective is covered with a sensitive coating which is 40% reflective. Writing is accomplished with a high intensity laser beam which burns a hole through the sensitive coating. This hole then shows up as a dark spot during a read operation since the nonreflective substrate is exposed. Typically, during a write pulse, the laser power increases from 1 milliwatt (mW) to above 5 mW. A write pulse thus gives at least a five times increase in intensity.
In addition to actual changes in the intensity, shape or position of the laser beam, apparent changes can result due to temperature drift of the photodetectors or other causes.
Typical methods for dealing with these variations are factory-set offsets and using an analog divider which divides the error signal by a signal which is a sum of the signals received from all of the photodetectors. Thus, changes in the intensity or changes due to temperature drift which give an apparent intensity change are compensated for since these terms appear in both the numerator and denominator of the ratio produced by the analog divider. The disadvantage of this method is the requirement for additional circuitry and the additional expense of a good analog divider. In addition, the analog divider does not compensate for unequal variations among the photodetectors.
The use of the same photodetectors for the error signal and the read signal presents additional problems. Unless the data is filtered, it may introduce uneven variations in the error signal. The filtering out of the data signal cannot be done until after an initial amplification, otherwise the data signal would be lost. Such systems require a wideband amplifier to preserve the data signal. Other systems use a simpler, narrowband amplifier, thus eliminating the cost and complexity of a wideband amplifier, by using a separate photocell for producing the data signal. An alternate method is to use a separate amplifier coupled across a resistor connected to the cathodes of the photodiodes to produce a separate data signal. The anodes of the photodiodes produce the signal used for error detection, which can thus use a simple amplifier.
There is always a need for improved optical disk servo systems which reduce complexity and cost and/or improve sensitivity.